Printing device using color profile calibration and method for operating a printing device using color profile calibration

ABSTRACT

The invention relates to a printing device and a method of operating the printing device, it being possible for the following steps to be carried out: printing a calibration print by means of the printing device, the calibration print having a plurality of measuring areas with different colour mixture relationships, spectrophotometric measurement of the measuring areas, producing a colour profile, the spectrophotometric measurement of the measuring areas being carried out at a plurality (N) of measuring times (T n ), producing a colour profile which is extended by a description of the behaviour over time and which comprises a plurality of colour profiles in each case associated with a measuring time, and setting the colour mixture relationships of the printing device on the basis of the colour profile extended by a description of the behaviour over time.

The invention relates to a printing device and a method of operating aprinting device. In particular, the invention relates to a printingdevice and a method of operating a printing device taking account of acolour change depending on the time which has elapsed since theprinting, by means of which obligatory measurement of colour values ofthe printing device can be carried out.

When prints are being prepared on printing devices, in particular inkjetprinters, a stable, predictable and controllable ink behaviour is veryimportant in many cases. This is the case in particular in the use inthe simulation of printing presses and in fine art reproduction or thereproduction of photographs.

It is known to perform the adaptation of the colour behaviour via whatis known as colour management. One example of this is formed by the useof what are known as ICC colour profiles established by the ICC(International Colour Consortium) in the printing industry. In thiscase, the colour behaviour of the printer on a specific material isdescribed by printing out a specific number of measuring areas withdifferent colour mixture relationships of the printer inks. Thesemeasuring areas are measured spectrophotometrically in order todetermine the colour behaviour of the printer. The measured valuesobtained in the process are then stored in what is known as a colourprofile, which reproduces the colour impression of the respectivemeasuring area. By using this colour profile, it is possible for colourmanagement software to determine that amount of the individual printerinks whose mixture on the measured material results in a desired colourimpression. The individual colours used by the inkjet printer are thenapplied to the material to be printed in accordance with these mixturerelationships.

In this case, however, the problem arises that the colours used ininkjet printers during printing react with the printed material and alsowith one another because of their chemical and microstructuralproperties. This also results in a change in the colour properties,which is most highly pronounced immediately after printing out. As hasbeen verified by long-term tests, however, a significant change in thecolour properties is also present after days and even weeks.

The deviation of the measured colour profiles from the ultimate colourimpression can in no way be neglected, in particular during the firsthalf an hour, for which reason the producers of colour managementsystems recommend waiting for at least 30 minutes before the measurementof the aforementioned measuring areas is carried out.

A further problem is that the above-described colour fluctuation neitherruns linearly nor in any other way in a specific direction. It istherefore possible, for example, for the case to arise in which, fromthe time of the print-out, the deviation of the colour values from thefinal result initially becomes greater for a specific time period andthen decreases slowly again until the colour values have finally largelystabilized. Because of this, the prints are initially kept for a certaintime before they can be measured.

This problem is particularly serious in applications in which a reliablemeasurement is virtually impossible as a result of the colourfluctuations during drying. This includes, for example, colourmeasurement directly during printing, which is desirable, for example,for on-line monitoring of the printing process.

Each arrangement of a colour measuring instrument directly on theprinter can carry out only a relative measurement. The result of thisrelative measurement normally deviates sharply from the result of ameasurement carried out in the same area a specific time period afterthe printing process, so that reliable statements using measured resultswhich have been obtained during or immediately after printing areproblematical.

The problem of a lack of agreement between colour measurement results isparticularly serious in the following cases:

-   -   a) if mutually different combinations of printer, ink and        material are concerned, for example because of different        manufacturers or different technologies (e.g. “piezo” versus        “bubblejet”);    -   b) if, as a result of the constructional arrangement of the        measuring instrument in or on the printer, different time        periods elapse between printing and measurement;    -   c) if a difference in the time of the measurement after the        printing is based on the dependence on the contents of the        printing data (hi/low resolution, slow/fast printing mode,        different dimensions of the image and, as a result, different        printing time). This is the case, for example, when the        measuring instrument is arranged on the printer but not on the        print head itself.

It is therefore an object of the present invention to provide a methodof operating a printing device and also a printing device in which theabovementioned disruptive influences are eliminated to the greatestpossible extent during the adaptation of the colour behaviour.

This object is achieved according to the features of the independentClaim 1.

For this purpose, a method of operating a printing device comprises thefollowing steps:

-   -   printing a calibration print by means of the printing device,        the calibration print having a plurality of measuring areas with        different colour mixture relationships;    -   spectrophotometric measurement of the measuring areas, producing        a colour profile, the spectrophotometric measurement of the        measuring areas being carried out at a plurality of measuring        times, producing a colour profile which is extended by a        description of the behaviour over time and which comprises a        plurality of colour profiles in each case associated with a        measuring time; and    -   setting the colour mixture relationships of the printing device        on the basis of the colour profile extended by a description of        the behaviour over time.

By measuring the colour values at regular time intervals after printing,the colour behaviour over time is characterized and this characteristicis added in a suitable way to the “normal” colour profile. In this, way,from a colour measurement and the knowledge of the time differencebetween printing and measurement, it becomes possible to calculate thecolour values at any other desired time. In practical terms, this othertime should be so long after printing that there are no longer anycolour changes worth mentioning, the colour is therefore stable.

In this way, it becomes possible for the first time to compare with oneanother measured results which have been measured at different timesfollowing printing and/or have been created with different combinationsof printer, ink and material and have been measured before stabilizationof the colour. This is possible in that, by using the timecharacteristic, all the measured values are converted to a time at whichthe colour values have stabilized. In particular, by means of the methodaccording to the invention, it therefore also becomes possible toachieve reliable results in the case of on-line monitoring of a printingprocess.

According to a preferred embodiment, the measuring times of theplurality of measuring times, that is to say their time intervals, arechosen such that the time interval between successive measuring timesincreases with the time since the calibration print was printed out. Theplurality of measuring times is preferably chosen such that the timeinterval between successive measuring times exhibits a logarithmicdependence on the time since the calibration print was printed out,preferably in accordance with the natural logarithmic function(“logarithmus naturalis”=ln). In this way, firstly the highest possibleaccuracy during the initial time period during which the measured colourprofiles vary relatively sharply over time is ensured. Secondly, bymeans of a relatively low number of measurements at a relatively largetime interval from the calibration print, it is ensured that only theleast possible storage space is needed for the total number of measuredcolour profiles.

The colour profile extended by a description of the behaviour over timepreferably has the colour profiles produced at the individual measuringtimes in each case with a time index which is greater the shorter thetime since the calibration print was printed out.

According to a further preferred embodiment, before the setting of thecolour mixture relationships by using the colour profiles associatedwith the individual measuring times and belonging to the colour profileextended by a description of the behaviour over time, further colourprofiles at times other than those actually measured are determined byextrapolation and/or interpolation. In this way, future colour values ofa print, that is to say the colour impression brought about by theprint, can be predicted as a function of time.

According to a further preferred embodiment, colour mixturerelationships of the printing device are set by using the extendedcolour profile in such a way that, after a specific time period haselapsed, predetermined colour values of the print are obtained. In thisway, an optimal print can be produced, taking into account the changeover time explained above of the colour impression.

According to a further preferred embodiment, the age of a print isdetermined by measuring a colour profile of the print and calculatingthe associated colour value by using the extended colour profile.

The printing device chosen is preferably a colour inkjet printer.

In a printing device according to the present invention, colour mixturerelationships of the printing device can be set by means of a colourprofile which is produced by spectrophotometric measurement of measuringareas of a calibration print having a plurality of measuring areas withdifferent colour mixture relationships, the means for spectrophotometricmeasurement being designed such that the spectrophotometric measurementof the measuring areas can be carried out at a plurality of measuringtimes, producing a colour profile which is extended by a description ofthe behaviour over time and which comprises a plurality of colourprofiles in each case associated with a measuring time, and wherein saidcolour mixture relationships of the printing device are adjustable onthe basis of the colour profile extended by a description of thebehaviour over time.

Further refinements of the invention can be gathered from thedescription and the subclaims.

The invention will be explained in more detail below using an exemplaryembodiment illustrated in the appended figures, in which:

FIG. 1 shows a graphical representation of measuring times T_(n) ofsuccessive measurements n (n=1, . . . 10) of colour profiles to producean extended colour profile according to a preferred exemplary embodimentof the method according to the invention in a single-logarithmic plot(FIG. 1 a) and a tabular listing of these measuring times T_(n) (FIG. 1b), in each case for T_(n)=15.e^(n-1), n=1, . . . 10; and

FIG. 2 shows the measuring times T_(n) of successive measurements n ofcolour profiles from FIG. 1 in a linear plot.

At the start of the method according to the invention, a calibrationprint is printed out in a known way by means of a printer, thecalibration print comprising a plurality of measuring areas withdifferent colour mixture relationships. The method is suitable inparticular for the obligatory measurement of colour values on colourinkjet printers, but can also be used for laser printers, for example.

In a next step, the individual measuring areas of the calibration printare measured spectrophotometrically, a colour profile being producedwhich, for each colour mixture relationship, comprises the colourimpression produced in the form of a spectral value. The mixturerelationships are preferably specified on the basis of what are known asthe four process colours cyan, magenta, yellow and black. However,mixture relationships based on more or fewer colours, in particularincluding only black and white, can also be used.

The colour profile can be drawn up in particular as an ICC colourprofile, which substantially comprises two tables. In a first table ofthe ICC colour profile, for each of the colour mixture relationshipsCMYK present in the individual measuring areas, the associated colourvalue in the Lab colour space Lab=Lab(CMYK) is specified. For eachcolour value Lab, a second table specifies the colour mixturerelationship CMYK needed to produce this colour value, on the basis ofpredetermined reference points of the inverse mathematical functionCMYK=CMYK (Lab).

The spectrophotometric measurement of the measuring areas is carried outrepeatedly at measuring times at predetermined time intervals. In thiscase, at each measuring time in each case a colour profile associatedwith this measuring time is produced.

According to the exemplary embodiment illustrated in FIG. 1 and FIG. 2,these measuring times are chosen such that the time intervals betweenthe measuring times increase logarithmically as a function of the timesince the time at which the calibration print of the printer wascompleted, specifically preferably in accordance with the functionT _(n) =k·e ^(n-1), (n=1, . . . , N; k=const.)   (1)where T_(n) designates the time period (in minutes) until the nthmeasurement, N designates the total number of measurements carried outin order to produce the extended colour profile, and k designates alinear correction factor.

In order to obtain a suitable relation between the size of the fileproduced for the extended colour profile, on one hand, and the number ofmeasured values on which subsequent interpolation or extrapolation isbased, a value approximately of the order of magnitude of ten shouldpreferably be chosen for the total number N of measuring points.

If, furthermore, the time period T_(n) until the chronologically lastmeasurement (n=10) is defined as a suitable value of T₁₀=12 weeks=120960minutes (such a time period corresponds approximately to the minimumkeeping time normally required for a colour proof), then, according tothe illustration in FIG. 1 a, given a logarithmic plot of T_(n) versus n(with n=1, . . . 10), a linear representation is obtained if the linearcorrection factor k=120960/e¹⁰⁻¹≈15 is chosen, so that the result is therelationship T_(n)=15·e^(n-1) (n=1, . . . , N; k=const.).

In this case, according to FIG. 1 a, the graphs shown in thesingle-logarithmic plot (that is to say with a logarithmic division ofthe T_(n) axis) specify the respective time period T_(n) in minutes(graph 10), hours (graph 20), days (graph 30) and weeks (graph 40). Theassociated values T_(n) for the individual measurements n=1, . . . , 10are listed in tabular form in FIG. 1 b.

As can be seen from the plot selected in FIG. 2, likewise for theabovementioned formula, the time periods between the individualmeasurements increase as the time from the chronologically firstmeasurement (n=1) increases, that is to say the starting time at whichthe calibration print was completed.

Because of the division of the timescale for the measuring points to beset for the colour profiles in accordance with Equation (1), that is tosay in accordance with the natural logarithm function, account is takenof the fact that the colour change of the inks used on the printedmaterial is at its most pronounced directly after the calibration printand subsequently slows. By means of a logarithmic arrangement of theindividual measuring times, firstly a higher accuracy is ensured in thetime period directly following the production of the calibration printand, at the same time, by selecting larger time intervals between themeasuring times in the chronological range of greater stability, thenecessary storage space is minimized.

Alternatively, but less preferably, instead of the above division of thetimescale for the individual measurements of the colour profiles,another nonlinear division of the timescale for the measuring points tobe set can be chosen. Possible in principle, but less suitable becauseof the steeper rise of the function, is also a division of the timescalein accordance with the decimal logarithm T_(n)=k·10^(n-1) (with n=1, 2,. . . , N), a measurement of the respective colour profile then beingcarried out after one minute (n=1), 10 minutes (n=2), 100 minutes (n=3),1000 minutes (n=4), etc.

Following completion of the measurements of the individual colourprofiles at the respective measuring times T_(n) (with n=1, . . . 10),the colour profiles produced at each measuring time are combined to forma colour profile which is extended by a description of the behaviourover time and which comprises the time period which has elapsed sincethe calibration print was completed as a further dimension in anadditional address region.

In this case, the individual colour profiles are preferably stored witha time index which is greater the shorter the time since the completionof the calibration print by the printer. The table section with thehighest time index therefore corresponds to the first spectrophotometricmeasurement prepared after the completion of the calibration print,while the chronologically last measurement is stored with the time indexzero.

This form of extended colour profile has the advantage that the extendedcolour profile can also be processed by means of a conventional methodsequence developed for the interpretation of ICC profiles without atimescale. In this case, the values of the extended colour profile whichhave been stored with the time index zero are preferably used, sincethese values correspond to the best approximation to he time at whichthe colour values are largely constant.

Following the completion of the extended colour profile, the latter isused for carrying out colour management, that is to say the setting ofthe colour mixture relationships of the printer is carried out on thebasis of the colour profile extended by a description of the behaviourover time.

By means of the extended colour profile, it is also possible for thosecolour values which are achieved on the same system at any otherarbitrary time to be calculated in a straightforward manner. This may beachieved by means of interpolation with adjacent reference points fromcolour values which have been measured at a specific time, by using theextended colour profile. During this interpolation, the individualcolour profiles present in the extended colour profile are weighted inaccordance with the logarithmic dependence on the measured points, thatis to say in accordance with Equation (1), in order to calculate thecolour value sought. For example, a colour value to be expected at atime T_(x) with T₇<T_(x)<T₈ can be determined with appropriate weightingof the colour values Lab(T₇) and Lab(T₈) measured at the times T₇ andT₈.

Furthermore, the age of a print can be determined by using the extendedcolour profile in that, by measuring a colour profile of the print andcalculating the associated colour value Lab(T_(y)), the correspondingtime period T_(y) which has elapsed since the completion of the print isdetermined by using the extended colour profile (for example bycomparison with a colour value stored therein at a different time, forexample T_(N) or the colour value for t→∞).

In addition, as early as when producing a print, account can be taken ofthe problem of the change of the colour values over time by the mixturerelationship already being chosen at the time of the print-out by usingthe extended colour profile in such a way that a desired colour valueLab (T_(z)) is obtained after a predetermined time interval T_(z) haselapsed.

Furthermore, it becomes possible to compare colour measurement resultsof prints on various printers with one another, to be specificirrespective of how much time has elapsed between the print-out and themeasurement. In particular, the colour values from various printerswhich have been measured at different times after the print-out can becalculated back to a common time base and compared in this way. Thismakes it possible in particular to obtain reliable statements about theprinting behaviour by means of on-line monitoring of the printingprocess.

1. Method of operating a printing device, which comprises the followingsteps: printing a calibration print by means of the printing device, thecalibration print having a plurality of measuring areas each withdifferent colour mixture relationships; spectrophotometric measurementof the measuring areas, producing from the measurement results a colourprofile, the spectrophotometric measurement being carried out at aplurality (N) of measuring times (T_(n)), wherein the measuring times(T_(n)) are chosen such that the time interval between successivemeasuring times increases with the time since the calibration print wasprinted out; wherein said color profile is produced as an extendedcolour profile which comprises a description of the behaviour over timeof the calibration print: wherein the extended colour profile comprisesa plurality of color profiles each being associated with a measuringtime (T_(n)); and setting the colour mixture relationships of theprinting device on the basis of the extended colour profile.
 2. Methodaccording to claim 1, the plurality (N) of measuring times (T_(n)) beingchosen such that the time interval between successive measuring timesexhibits a logarithmic dependence on the time since the calibrationprint was printed out.
 3. Method according to claim 2, a naturallogarithmic function being chosen as the logarithmic dependence. 4.Method according to claim 1, wherein the colour profile extended by adescription of the behaviour over time having the colour profilesproduced at the individual measuring times (T_(n)) in each case with atime index which is greater the shorter the time since the calibrationprint was printed out.
 5. Method according to claim 1, furthercomprising determining additional colour profiles by extrapolationand/or interpolation.
 6. Method according to claim 1, wherein colourmixture relationships of the printing device being set by using theextended colour profile in such a way that, after a specific time periodhas elapsed, predetermined colour values of the print are obtained. 7.Method according to claim 1, wherein the age of a print being determinedby measuring a colour profile of the print and calculating theassociated colour value by using the extended colour profile.
 8. Methodaccording to claim 1, wherein a colour inkjet printer being chosen asthe printing device.
 9. Printing device in which colour mixturerelationships of the printing device can be set by means of a colourprofile, said printing device comprising means for spectrophotometricmeasurement of measuring areas of a calibration print having a pluralityof measuring areas with different colour mixture relationships forproducing the colour profile, the means for spectrophotometricmeasurement being designed such that the spectrophotometric measurementof the measuring areas can be carried out at a plurality (N) ofmeasuring times (T_(n)), producing a colour profile which is extended bya description of the behaviour over time and which comprises a pluralityof colour profiles each being associated with a measuring time (T_(n)),and such that the time interval between successive measuring timesincreases with the time since the calibration print was printed out; andsaid colour mixture relationships of the printing device are beingadjustable on the basis of the colour profile extended by a descriptionof the behaviour over time.
 10. Printing device according to claim 9,the means for spectrophotometric measurement being designed such thatthe time interval between successive measuring times exhibits alogarithmic dependence on the time since the calibration print wasprinted out.
 11. Printing device according to claim 10, the logarithmicdependence being described by a natural logarithmic function. 12.Printing device according to claim 9, the printing device being a colourinkjet printer.
 13. Printing device according to claim 9, the printingdevice being a colour inkjet printer.
 14. Printing device according toclaim 10, the printing device being a colour inkjet printer. 15.Printing device according to claim 11, the printing device being acolour inkjet printer.
 16. Method according to claim 1, wherein thecolour profile extended by a description of the behaviour over timehaving the colour profiles produced at the individual measuring times(T_(n)) in each case with a time index which is greater the shorter thetime since the calibration print was printed out.
 17. Method accordingto claim 2, wherein the colour profile extended by a description of thebehaviour over time having the colour profiles produced at theindividual measuring times (T_(n)) in each case with a time index whichis greater the shorter the time since the calibration print was printedout.
 18. Method according to claim 3, wherein the colour profileextended by a description of the behaviour over time having the colourprofiles produced at the individual measuring times (T_(n)) in each casewith a time index which is greater the shorter the time since thecalibration print was printed out.
 19. Method according to claim 1,further comprising determining additional colour profiles byextrapolation and/or interpolation.
 20. Method according to claim 2,wherein before the setting of the colour mixture relationships by usingthe colour profiles associated with the individual measuring times(T_(n)) and belonging to the colour profile extended by a description ofthe behaviour over time, further colour profiles at times other thanthose actually measured being determined by extrapolation and/orinterpolation.
 21. Method according to claim 3, further comprisingdetermining additional colour profiles by extrapolation and/orinterpolation.
 22. Method according to claim 4, further comprisingdetermining additional colour profiles by extrapolation and/orinterpolation.
 23. Method according to claim 1, wherein colour mixturerelationships of the printing device being set by using the extendedcolour profile in such a way that, after a specific time period haselapsed, predetermined colour values of the print are obtained. 24.Method according to claim 2, wherein colour mixture relationships of theprinting device being set by using the extended colour profile in such away that, after a specific time period has elapsed, predetermined colourvalues of the print are obtained.
 25. Method according to claim 3,wherein colour mixture relationships of the printing device being set byusing the extended colour profile in such a way that, after a specifictime period has elapsed, predetermined colour values of the print areobtained.
 26. Method according to claim 4, wherein colour mixturerelationships of the printing device being set by using the extendedcolour profile in such a way that, after a specific time period haselapsed, predetermined colour values of the print are obtained. 27.Method according to claim 5, wherein colour mixture relationships of theprinting device being set by using the extended colour profile in such away that, after a specific time period has elapsed, predetermined colourvalues of the print are obtained.
 28. Method according to claim 1,wherein the age of a print being determined by measuring a colourprofile of the print and calculating the associated colour value byusing the extended colour profile.
 29. Method according to claim 2,wherein the age of a print being determined by measuring a colourprofile of the print and calculating the associated colour value byusing the extended colour profile.
 30. Method according to claim 3,wherein the age of a print being determined by measuring a colourprofile of the print and calculating the associated colour value byusing the extended colour profile.
 31. Method according to claim 4,wherein the age of a print being determined by measuring a colourprofile of the print and calculating the associated colour value byusing the extended colour profile.
 32. Method according to claim 5,wherein the age of a print being determined by measuring a colourprofile of the print and calculating the associated colour value byusing the extended colour profile.
 33. Method according to claim 6,wherein the age of a print being determined by measuring a colourprofile of the print and calculating the associated colour value byusing the extended colour profile.
 34. Method according to claim 1,wherein a colour inkjet printer being chosen as the printing device. 35.Method according to claim 2, wherein a colour inkjet printer beingchosen as the printing device.
 36. Method according to claim 3, whereina colour inkjet printer being chosen as the printing device.
 37. Methodaccording to claim 4, wherein a colour inkjet printer being chosen asthe printing device.
 38. Method according to claim 5, wherein a colourinkjet printer being chosen as the printing device.
 39. Method accordingto claim 6, wherein a colour inkjet printer being chosen as the printingdevice.
 40. Method according to claim 8, wherein a colour inkjet printerbeing chosen as the printing device.